With the development of power electronics technology, switching power supplies are tending to be miniaturized and lightweight. To reduce the volume and the weight of a power supply, the most feasible approach is to increase the switching frequency. Because of operation in a high frequency state, the effects of parasitic parameters such as the switching transformer leakage inductance, the distributed capacitance, etc. cannot be ignored. For example, in a liquid crystal display driving circuit, in a topology application based on a non-isolated DC to DC (DC/DC) converter, since power devices (which refer to electronic components which output relatively large power) work in a switching state, there exists a very large current change rate, and in a practical switching power supply circuit, there inevitably exist stray inductance and parasitic capacitance, so when working at full load, the voltage spike of the switching power supply will be very high and will take a ringing form, which will not only seriously affect the lifetime and reliability of devices in the circuit, but also cause some loss of efficiency, and meanwhile, high frequency, high di/dt and high dv/dt might bring about large electromagnetic interference (EMI for short). In a situation where the integration level of the liquid crystal display driving circuit is higher and higher and under the application requirements for high definition and high frequency, this problem is particularly prominent and urgently needs to be solved.
Nowadays, there are many approaches which may achieve the purpose of absorption, and in general, they are mainly through two ways, one is to reduce the leakage inductance, and one is to dissipate the energy of the overvoltage. The reduction of the leakage inductance mainly relies on the process, and the dissipation of the energy of the overvoltage is through the absorption circuit in parallel with the transformer and alternatively the switch transistor.
In conventional applications, based on cost and space conditions, the schemes used for absorption circuits are to connect an RC (resistor and capacitor) circuit (see FIG. 1) and alternatively an RCD (resistor, capacitor and diode) circuit (see FIG. 2) at both ends of the switch transistor in parallel. The basic working principle of these absorption circuits is to provide the switch transistor with a bypass when the switch transistor is disconnected, so as to absorb the energy accumulated in the parasitic inductance, and cause the switch voltage to be clamped, thereby suppressing the voltage spikes. Although these schemes have a certain effect and may reduce the magnitude of the spike voltage of the switching power supply, the energy of the reduced spike voltage is heat converted into a large amount of heat by the resistor(s) in the circuit, which lower the conversion efficiency of the switching power supply.